The present invention relates to a press section without press suction rollers in a paper machine.
In the press section, a paper web arriving from the wire section of the wire section of the paper machine is conducted, supported by a first topside fabric, at least through a first two-fabric nip. The first fabric also serves in connection with the pick-up roller as the pick-up fabric. The press section comprises, in closed roller combination, at least three, and preferably four, press rollers defining press nips together, two and two. The two of these press rollers which are first in the web's direction of travel are press rollers provided with a solid recessed surface. The third press roller is a smooth-surfaced central roller of the press section. The fourth roller, if any, is a recessed surface press roller. The press section further comprises a second fabric serving as a pressing fabric at least in the first two-fabric press nip. The web is conducted through the first press nip defined by the first and second press rollers interposed between the first and second fabrics and to the second nip and therethrough, carried by the first or second fabric. The second press nip is defined between the second press roller and the third smooth-surfaced press roller, and the third press nip, if any, is provided with a press fabric of its own.
The starting point of the invention was to develop further the same applicant's "Sym-Press" (Finnish Trademark) press section based on the service experience gained with the "Sym-Press" press over a period of several years.
The details of the "Sym-Press" press section are disclosed in Finnish publication document No. 50651 of the same applicant. The "Sym-Press" press section is a compact, so-called completely closed, press section wherein the paper web coming from the wire is conducted through a first nip defined by a recessed surface roller and a suction roller, interposed between two felts. The paper web is dewatered through both its surfaces. This press section comprises a smooth-surfaced central roller provided with at least one doctor means. A second nip is defined against this central roller, together with the aforementioned suction roller. Dewatering occurs in this nip through that surface of the paper web which faces the second roller of the first nip. In addition, at least one additional nip follows the second nip in this press section. The additional nip is defined between the smooth-surfaced central roller, which is larger in diameter than any other press roller in the press section, and a recessed surface roller, and a felt running through the additional nip. The additional nip is located substantially on the side of the central roller opposite the second nip.
The state of art associated with the invention is further disclosed in U.S. Pat. No. 4,257,844 and in the magazine articles, "Das Papier", Heft 1, pages 33-34, 1981, and "Norsk Skogindustri" No. 3, 1974, page 80.
The "Norsk Skogindustri" article describes a modification of the aforementioned "Sym-Press" press section, in which the "Sym-Press" suction roller does not define a nip with the smooth-surfaced central roller. In this press section, a first double-felted press nip is arranged in conjunction with, or preceding, this suction roller. Dewatering occurs in two directions. A recessed surface press roller is disposed in conjunction with the smooth-surfaced central roller, substantially replacing the "Sym-Press" suction roller. The recessed surface press roller defines the second press nip of the press section together with the smooth-surfaced central roller. A third press nip is defined substantially on the opposite side of the smooth-surfaced central roller with reference to the second nip.
In known press sections, a press suction roller must be used, thereby introducing considerable drawbacks, as hereinafter discussed.
The perforation of the press suction roller may leave a marking on the web which is detrimental to the appearance of the paper and may affect the surface characteristics of the paper. Press suction rollers are expensive. They require an individual drive motor and a control system. Furthermore, they are noisy. In particular, their drilling is a demanding task and involves high costs. The perforation reduces the strength of the shell, so that special metal alloys have to be used as the raw material of the rollers, and the thickness of the shell must be great, resulting in high costs.
Press suction rollers consume much air, due to the fact that not only the air passing through the web and the felt enters the suction system, but also the air which enters the suction zone in the holes in the suction roller's shell upon each revolution. In addition, the sealing water of the suction box of the roller causes much trouble.
A drawback of operating technology associated with suction rollers is that the suction rollers make considerable noise.
Although it is often necessary to arrange for deflection compensation in press suction rollers particularly, this is usually not possible, because the shell of the suction roller is perforated and/or because the interior volume of the roller is occupied by the suction box to such an extent that, as a result, deflection compensating means, known in themselves in the art, cannot be accommodated therein.
As hereinbefore stated, the press suction roller in particular is an extremely expensive component. At present price levels it may cost as much as FIM 3 million.
U.S. patent application No. 470,197, filed Feb. 28, 1983, discloses the state of art most closely related to the invention. This Finnish patent application was filed on Mar. 5, 1982 and discloses a press section with objects largely similar to those hereinbefore mentioned.
In the press section disclosed in U.S. patent application No. 470,197, it is considered novel that:
(a) the press section has no press suction rollers, since its first double-felted press nip is defined between two recessed surface rollers having solid shells, PA1 (b) after or before the first nip (FIG. 7) the run of one fabric, that is, the lower fabric, and simultaneously the travel of the web supported by this fabric, is turned downward prior to the second, single-felt nip under guidance, through a given sector, by a suction roller or a recessed surface press roller within the loop of the second fabric. The magnitude of the sector is in the range of about 30.degree. to 160.degree., and PA1 (c) the web is so conducted through the nips, at least three in number, that that surface of the web which was the surface opposite the forming wire will in the second and third press nips lie against the smooth surface of the central roller of the press.
Finnish patent application No. 763434 of the same applicant discloses the state of art closely related to the invention. This Finnish patent application is equivalent to U.S. Pat. No. 4,192,711 and discloses a procedure for detaching the paper web at the wet end of a paper machine from the forming wire, for its conduction in so-called closed conduction to the press section and for performing a dewatering pressing operation. In the procedure of U.S. Pat. No. 4,192,711, the web is conducted to lap a grooved and/or perforated roller within the felt loop on a very wide sector. Steam treatment is provided, with the steam being directed at the web from outside. The web and the felt, if any, under it on the roller is heated by the steam and the web is supported from outside as its direction changes. Thereafter, the heated web is carried on the felt to the first press nip, where it is pressed between a recessed surface roller and a smooth-surface roller in order to dewater it.